Switching mode DC-to-DC power converters are widely used in power supply circuits, in which the currents of each phase are accurately sensed, and it is therefore very important to balance the currents between each phase. When a conventional switching mode DC-to-DC power converter desires to generate a current sense signal for the purpose of current balance thereof, typically it utilizes an external sense resistor, such as a power resistor of low resistance additionally connected in series, the conductive resistance of a power component, or the ESR (Equivalent Series Resistance) of an inductor or capacitor. FIG. 1 shows a conventional current sense apparatus 10 for one phase of a switching mode DC-to-DC power converter that has a high-side MOS 102 connected between a high voltage power supply Vin and an output node 106, a low-side MOS 104 connected between the output node 106 and ground, an inductor 108 conected to the output node 106 to derive the output current IL and output voltage Vo, and both load capacitor 112 and resistor 114 connected to the converter output 116, and for the purpose of current sensing, a sense resistor 110 is inserted between the inductor 108 and converter output 116 for the output current IL to flow therethrough, in combination with an operational amplifier 118 to amplify the voltage drop across the sense resistor 110 to generate a current sense signal VIS. However, a DC value proportional to the output current IL is generated from the DC value of the voltage drop across the sense resistor 110 and thereby introduces a regulation error to the output voltage. Moreover, due to the switching noise resulted from the parasitic noise element in the system, the SNR (Signal-to-Noise Ratio) is very low for the AC componant of the voltage drop across the sense resistor 110, and the error in the slope of the measured inductor current caused by this noise may result in unstability and failure to the power converter. In addition, the sense resistor 110 consumers electric power and subsequently reduces the efficiency of the converter.
In order to prevent a switching mode DC-to-DC power converter from the above-mentioned problems, an apparatus and method was proposed by U.S. Pat. No. 6,377,032 issued to Andruzzi et al., which simulates the current sense signal using three current sources to approximate or virtualize the real output current of the power converter. In detail, to generate the ripple of the simulated signal, a first current source proportional to the difference between the input and output voltages is used to charge a current sense capacitor to simulate the rising portion of the real signal, and a second current source proportional to the output voltage is used to discharge the current sense capacitor to simulate the falling portion of the real signal. Also, a third current source proportional to the output voltage is used to charge a ramp capacitor and a switch is used to control the charging and discharging of the ramp capacitor to generate a ramp waveform. The ripple and ramp waveforms are then combined to become the current sense signal that is approximately the inductor current of the power converter. However, this circuit is complicated and the current sense signal generated thereof has no physical meaning since it is a virtual signal or one obtained by way of simulations. It is therefore desired a current sense apparatus and method implemented by simpler circuit to generate the current sense signal almost as real as the output current of a switching mode DC-to-DC power converter.